Because bass frequencies are not directional like higher frequencies, and the fact that the sub and LFE tracks are mono, this is treated as one channel. The EQ will treat it as the same. Both subwoofers will be pumping out your test tone and because they are not directional, or stereo, you will EQ them as one channel to provide room correction at the listening position.

I suspect most people, rather than buy an interface, will want to connect either their computer's headphone or line level output directly to the Model 976's analog 7.1 channels - one at a time and then run REW from their computer through the Model 976 to correct each speaker.

Also, rather than buy a mic pre, you could use a USB mic hooked directly to your computer to interface with REW. REW's EQ window will then let us see what filters you need to apply to address any problems you have.

REW's database is pretty good if you are interested in delving into this before the product is released: https://www.roomeqwizard.com/help.html
I think you might find #5 and #32 helpful.